1. Field of the Invention
The present invention relates to a rechargeable lithium battery incorporating a positive electrode comprised of hexagonal-structured lithium molybdenum oxide active material and also to a positive electrode material (active material of a positive electrode) for rechargeable lithium batteries.
2. Description of Related Art
The molybdenum oxides conventionally known as useful positive electrode materials for rechargeable lithium batteries are MoO2 and MoO3. However, these positive electrode materials undergo significant changes in crystal structure with repetitive charge-discharge cycling, which has been a problem.
In order to overcome the problem, the use of Li2MoO3, which has a more stable hexagonal crystal structure, for positive electrode material have been investigated (See, for example, Extended Abstracts, Electrochem. Soc. Fall Meet, 1998, p. 207). Hexagonal-structured Li2MoO3 has a space group R{overscore (3)}m where Li is present at 3a sites, Mo and Li at 3b sites and O at 6c sites. A major proportion of Li present at 3b sites is not stored or released during charge and discharge, which contributes to the holding or retention of crystal structure. This conceivably suppresses the change in crystal structure with charges and discharges.
However, the stability in crystal structure of Li2MoO3 has been still insufficient. Therefore, there is a need for a positive electrode material which results in the provision of improved charge-discharge cycling characteristics.
It is an object of the present invention to provide a rechargeable lithium battery which exhibits improved charge-discharge cycling characteristics and a positive electrode material for use in rechargeable lithium batteries.
The rechargeable lithium battery of the present invention has a positive electrode containing hexagonal-structured lithium molybdenum oxide as active material. The lithium molybdenum oxide, on charge and discharge, is represented by a formula LixMyMo1xe2x88x92yO3, where M is at least one element selected from the group consisting of Al, Ti, V, Mn, Fe, Co, Ni and Nb, x satisfies the relationship 0.7xe2x89xa6xc3x97xe2x89xa62.2 and can be varied within this range by storage and release of lithium ions during charge and discharge, and y satisfies the relationship 0 less than yxe2x89xa60.50.
In the present invention, the element M which has been substituted for a part of molybdenum in lithium molybdenum oxide, i.e., either one of the above-listed eight elements, can exist in the form of a trivalent or tetravalent ion having an ionic radius of 0.050-0.070 nm in six-coordination (Acta Cryst., A32, pp. 751-767). Thus the element M is similar in property to molybdenum (Mo).
As stated above, the positive electrode material of the present invention is characterized as containing the lithium molybdenum oxide in which Mo has been partly replaced by a specific element-such as Al. In the above-specified compositional formula, x satisfies the relationship 0.7xe2x89xa6xc3x97xe2x89xa62.2. The departure of x from this specified range may result in a marked reduction of charge-discharge cycling characteristics. Also, y satisfies the relationship 0 less than yxe2x89xa60.50. A value of higher than 0 is assigned to y because when molybdenum is partly replaced by the substituting element, a bond strength between oxygen located at 6c sites and the elements located at 3b sites of crystal structure, i.e., the substituting element and remaining molybdenum, is increased to stabilize the crystal structure, resulting in obtaining the effect of the present invention. Also, a value of y is not permitted to exceed 0.50 because for y greater than 0.50, a crystal structure is hardly maintained in hexagonal form and becomes unstable. As will be discussed later, it is more preferred that y satisfies the relationship 0.10xe2x89xa6yxe2x89xa60.40.
In the compositional formula LixMyMo1xe2x88x92yO3, the oxygen stoichiometry is 3. However in actual cases, the oxygen stoichiometry is varied generally within the range of 2.8-3.2 depending upon the calcining temperature and period used in the preparation of the lithium molybdenum oxide. For the purpose of the present invention, the lithium molybdenum oxide as represented by the compositional formula LixMyMo1xe2x88x92yO3 encompasses the cases where the oxygen stoichiometry is varied within the above-specified range.
The element M present in the lithium molybdenum oxide for use in the present invention can be selected from the aforementioned eight elements. Preferably, M may be at least one element selected from the group consisting of Mn, Fe, Co and Ni. The use of any of these four elements for the element M further suppresses the change in crystal structure with charge-discharge cycling, resulting in further improving charge-discharge cycle characteristics. Therefore, the particularly preferred for use in the present invention is the lithium molybdenum oxide of the above-specified compositional formula where M is at least one element selected from the group consisting of Mn, Fe, Co and Ni, and y is between 0.10 and 0.40. If these conditions are properly satisfied, the change in crystal structure with charge-discharge cycling is further suppressed to result in further improved charge-discharge cycle characteristics.
The positive electrode for use in the present battery can be fabricated by utilizing the aforementioned lithium molybdenum oxide as active material. Generally, a powder of lithium molybdenum oxide, optionally in combination with a binder and/or an electrical conductor, is pressed in mold into shapes. The lithium molybdenum oxide powder has a specific surface area preferably in the range of 0.5-6.0 m2/g, more preferably in the range of 1.0-5.0 m2/g, as determined by a BET""s adsorption method using a nitrogen gas. If the BET specific surface area falls below the above-specified range, the excessively small specific surface area of the positive electrode material may lower the charge-discharge reactivity thereof. On the other hand, if the BET specific surface area goes beyond the above-specified range, the positive electrode material, because of its excessively high reactivity, may be caused to partly dissolve into an electrolyte solution.
An important feature of the present invention resides in its utilization of a specific positive electrode material for providing rechargeable lithium batteries which exhibit excellent charge-discharge cycle characteristics. For other complementary components, such as a negative electrode and an electrolyte, any materials conventionally known in the art can be used, for example.
Examples of negative electrode materials include metallic lithium; lithium alloys such as a lithium-aluminum alloy, lithium-lead alloy and lithium-tin alloy; carbon materials such as graphite, coke and calcined organic substances; and metal oxides, such as SnO2, SnO, TiO2, which have lower electric potentials compared to the positive electrode materials.
Examples of useful non-aqueous electrolyte solvents include high-boiling solvents such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonates (VC) and butylene carbonate (BC); and mixed solvents of any of those high-boiling solvents and one or more of the following low-boiling solvent: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,2-diethoxyethane (DEE), 1,2-dimethoxyethane (DMC) and ethoxymethoxy ethane (EME).
Examples of suitable non-aqueous electrolyte solutes include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2) (C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3 and any combination thereof.
The present battery includes a positive electrode containing, as its active material, the aforementioned lithium molybdenum oxide which undergoes little change in crystal structure during storage and release of lithium ions, so that the reduction of discharge capacity with charge-discharge cycling can be effectively suppressed. Therefore, in accordance with the present invention, a rechargeable lithium battery can be provided which exhibits excellent charge-discharge cycle characteristics.
The positive electrode material of the present invention contains the lithium molybdenum oxide having a hexagonal structure and is particularly suitable for use in rechargeable lithium batteries. Characteristically, molybdenum in the lithium molybdenum oxide has been partly substituted by the metal which is at least one element selected from the group consisting of Al, Ti, V, Mn, Fe, Co, Ni and Nb.
Preferably up to 50 atomic %, more preferably between 10 and 40 atomic % of an initial content of molybdenum has been substituted. Specifically, any of the above-specified elements can be substituted for a part of molybdenum in lithium molybdenum oxide, for example, represented by the compositional formula Li2MoO3. It should be recognized that the lithium and oxygen stoichiometries will be varied depending upon the blending proportions of materials used, calcining temperature, calcining period and the like.
More specifically, the positive electrode material of the present invention contains the lithium molybdenum oxide, as represented by the aforementioned compositional formula LixMyMo1xe2x88x92yO3, which has been previously described as being useful active material for the rechargeable lithium battery of the present invention.
The use of the positive electrode material of the present invention effectively suppresses the reduction of discharge capacity with repetitive charge-discharge cycling, resulting in the provision of rechargeable lithium batteries which exhibit improved charge-discharge cycle performance characteristics.